Tile settings: 1024 x 1024
Min Samples: 10
Max Samples: 50
Noise Threshold: 0.05
Time Limit: 0
Render size: 2x 1080P (4K).
When using the following settings (simply toggle Tiled Rendering ON / OFF to test):
The difference in final render Noise Levels is significant as demonstrated by the attached files below. Attached so that they download and can be opened in an image editor easier.
It is almost as if the Noise Threshold value is ignored or not being picked up by one of the renders. I don't really know what is going on but the settings are the same other than toggling Use Tiles Rendering ON or OFF and pressing F12.
The sample scene is also included.
In the image below non tiled render is on the left and tiled render is on the right. (Vastly different noise levels).